In many fluid delivery systems, it is important to be able to carefully control the fluid flow rates. With respect to systems intended to be used in the intravenous administration of fluids, the precise control of fluid flow rates is usually a crucial part of the therapy being provided to the patient. In the medical field, then, accuracy is an important feature of a flow control system. Furthermore, the intravenous administration of fluids at uncontrolled high fluid flow rates can be harmful to the patient.
Another desirable feature of a flow control system is consistency over time. In the medical field, it is impractical to expect a medical attendant to be present to monitor the fluid flow rate during the entire treatment session. A flow rate control system must thereby be capable of maintaining a stable flow rate while unattended for relatively long periods of time.
Also in the medical field, as well as other environments, yet another desirable feature for a flow control system is simplicity to avoid operator error, which is more likely as the complexity of a device increases. The simplicity of a flow control system also has a bearing upon the overall cost of the system, which of course is to be minimized to the fullest extent possible without detracting from the quality of performance.
Many conventional intravenous fluid administration sets are provided with manual clamping devices, such as roller clamps, which provide a means for the operator to manually control the fluid flow rates. Roller clamps have long served well for this purpose. However, as medical treatments become more sophisticated and more precise, so too should the apparatus which administer these treatments. For example, since roller clamps are virtually infinitely variable between a fully open and a fully closed position, the operator must take time to carefully adjust the clamp to achieve the rate of flow desired. Often, a desired flow rate cannot be achieved with precision. Furthermore, the flow rate, once achieved, can vary due to the tendency of flexible tubing used in intravenous administration systems to "creep" under the compression of the clamp. The flow rate initially established at the outset of the treatment session can unexpectantly and unpredictably change during the course of treatment.
In portable drug administration systems (such as shown in PCT International Publication Number WO 861 03978, entitled Infusion Having a Distal Flow Regulator, which is assigned to the same assignee as this application), extruded glass capillary tubes are sometimes used to offer a fixed resistance to flow. However, while this control technique is relatively straightforward in its approach, it is costly to extrude a capillary tube having the desired precise flow resistance.
The need therefore still exists for cost effective systems to achieve stable and precise fluid flow rate control in a fluid administration system.